When a ball is hit with a golf club head whose face has a loft angle, a backspin occurs on the hit ball based on the friction force between the face and the ball. It has been believed that the loft angle and the friction coefficient of the face become greater, the backspin becomes increased.
However, in the case of golf club heads such as wood-type golf club heads whose loft angle is small, it has been known through various experiments that as the friction coefficient of the face becomes large, the backspin is reduced, and that as the friction coefficient of the face becomes small, the backspin is increased (see the following Patent Document 1). Such a phenomenon is believed to be caused by a recoil occurring in the interior of the ball.
The recoil is a type of resilience of a golf ball in which an elastic torsional deformation is caused by the contact with the club face, and the recoil returns the golf ball to its original state. In general, if the recoil occurs strongly, the backspin is reduced.
In the case of a golf club head whose loft angle is small, as the friction coefficient of the face becomes large, the recoil is relatively increased, and the backspin of the ball is reduced. In the case of a golf club head whose loft angle is small, as the friction coefficient of the face becomes small, the recoil is hindered, and the backspin of the ball is increased.
The following Patent Document 2 discloses a golf club head utilizing a phenomenon as described above, wherein an upper region of the face located upper than the sweet spot of the face has a ten-point average roughness being smaller than 1.5 micrometers, and a lower region of the face located lower than the sweet spot of the face has a ten-point average roughness of 1.5 to 10 micrometer.
When a ball is hit by the upper region of the face, usually, the backspin of the ball is reduced by a gear effect. However, in the wood-type golf club head disclosed in the Patent Document 2, as the upper region of the face is decreased in the friction coefficient, the action of the recoil is reduced, and it is possible to prevent a significant decrease in the backspin of the ball.
On the other hand, when a ball is hit by the lower region of the face, usually, the backspin of the ball is increased by a gear effect. However, in the wood-type golf club head disclosed in the Patent Document 2, as the lower region of the face is increased in the friction coefficient, the action of the recoil is strengthened, and it is possible to prevent a significant increase in the backspin of the ball.
Thus, in the golf club head according to the Patent document 2, it is possible to stabilize the flying distances of the hit balls even if the hitting position varies above and below the face.